1. Field of the Invention
The present invention relates to a sputtering cathode used in particular for a magnetron sputtering apparatus, a sputtering apparatus provided with the sputtering cathode, a film-forming method, and a method for manufacturing an electronic device.
2. Related Background Art
Conventionally, there have been proposed various types of sputtering cathode structures as those for sputtering apparatuses. A magnetron-based sputtering cathode, among these structures, has a high deposition rate and is, therefore, most often used industrially.
Conventionally, there have been various types of magnetron-based cathodes. At present, a sputtering apparatus equipped with a planar magnetron cathode provided with a planar-shaped target, among sputtering apparatus having a magnetron-based sputtering cathode, is industrially most useful.
A sputtering apparatus provided with a planar-shaped target is used to manufacture electronic devices mainly, such as semiconductors and electronic components. In the manufacture of liquid-crystal display (LCD) panels and solar cells, among other electronic devices, a sputtering apparatus provided with a rectangular magnetron-based sputtering cathode is used to form electrodes and interconnects. In recent years, there has been a growing demand, along with an increase in the size of LCD panels and solar cells, that a sputtering apparatus deposits thin films on a large-area substrate.
For example, there is mentioned a thin-film solar cell formed of a common compound semiconductor, as an example of electronic devices including a multilayer film.
As one example of the basic structure of the solar cell, there can be mentioned a structure in which an Mo electrode layer to serve as a back electrode (positive electrode) is film-formed on an SLG (soda-lime glass) substrate, a light-absorbing layer is deposited on the Mo electrode layer, and a transparent electrode layer made of ZnO:Al or the like and intended to serve as a negative electrode is deposited on the light-absorbing layer through a buffer layer made of ZnS, CdS or the like. These layers are respectively deposited by means of sputtering using a magnetron-based cathode.
In general, one or a plurality of planar rectangular magnet units are used for a planar magnetron cathode provided with a target having a planar rectangular shape. The rectangular-shaped magnet unit is configured by arranging a bar-like magnet intended to serve as a central magnetic pole and a peripheral magnetic pole surrounding the bar-like magnet on a yoke made of a soft magnetic substance. The polarity of the central magnetic pole and the polarity of the peripheral magnetic pole are made opposite to each other.
A technique to form a thin film on a large-area substrate is disclosed in Japanese Patent Application Laid-Open No. H8-199354. Japanese Patent Application Laid-Open No. H8-199354 discloses the configuration of a cathode in which a plurality of rectangular-shaped magnet units are used so that a spacing between respective magnet units can be adjusted independently, and a mechanism, whereby a distance between a target and a surface of a magnet can be adjusted, is provided on both ends of each magnet unit in the long-side direction (longitudinal direction) thereof.
As is obvious from the description given above, in film formation using a cathode provided with magnet units, it is possible to uniformize the density distribution of plasma in the longitudinal direction of a magnet unit. Accordingly, the method described in Japanese Patent Application Laid-Open No. H8-199354 is extremely effective. However, even such an effective configuration as described above has problems that remain to be solved in order to further improve a film thickness distribution.
In particular, in the configuration shown in Japanese Patent Application Laid-Open No. H8-199354, the mechanism whereby a distance between the target and the surface of a magnet can be adjusted is provided only at both ends of each magnet unit in the long-side direction thereof. Accordingly, the only way to adjust a film thickness distribution in the long-side direction of the magnet unit is to tilt the magnet. Thus, there has been the problem to be solved that restrictions are imposed on the degree of freedom of film formation, including the adjustment of the film thickness distribution.
Another problem is that a driving system for causing each magnet unit to make a sliding movement and the mechanism for adjusting a distance between the target and the surface of the magnet are integrated with each other. Accordingly, a mechanism for driving the magnet unit is complicated. Thus, the configuration has problems to be solved, including the problem that a difficulty arises in maintenance work and the like.
Yet another problem is that in the configuration described in Japanese Patent Application Laid-Open No. H8-199354, the driving system for causing each magnet unit to make a reciprocating movement and the mechanism for adjusting a distance between the target and the surface of the magnet are arranged on the atmosphere side. Although the inside of a sputtering chamber, which is the vacuum side, and the atmosphere side are isolated from each other by a backing plate for supporting the target, a high pressure is applied to the backing plate when the sputtering chamber is evacuated. Accordingly, increasing the sizes of the target and the backing plate along with an increase in the size of a substrate, which is an object to be film-formed, causes an increase in the amount of warpage of the target and the backing plate due to the pressure difference thereof from the atmospheric pressure. As a result, the film thickness distribution may become nonuniform due to the nonuniformity of magnetic field distribution on the target.